The valve actuation system of an internal combustion engine for opening and closing intake valves and exhaust valves typically uses rocker arms that are each provided with a cam engagement portion for engaging a cam formed on a camshaft and a valve engagement portion for engaging the stem end of a valve. The camshaft is rotatably supported by a cam holder provided on the cylinder head, and a rocker arm shaft that rotatably supports the rocker arm is fixed to the cam holder. In such a valve actuating mechanism, vibrations that are produced as a result of actuation of the valve are transmitted to the outside, and are emitted as noises. In particular, when variable mechanisms such as variable valve lift mechanism, variable valve timing mechanism, a variable compression ratio mechanism, etc. is used, the adjusting mechanism tends to apply an additional stress to various parts of the engine, and the vibration problems often become even more acute. The vibrations are transmitted via at least two paths. Firstly, the vibrations owing to the collision between the valve and the valve engagement portion of the rocker arm are transmitted to the cam holder via the rocker arm. Secondly, the vibrations owing to the collision between the cam and cam engagement portion of the rocker arm are transmitted to the cam holder via the rocker arm or camshaft. In either case, the vibrations transmitted to the cam holder are emitted to the atmosphere via the cylinder head, head cover and so on, and turn into noises.
The crankshaft system comprises a connecting rod that transmits the reciprocating movement of the piston, a crankshaft that converts the movement transmitted from the connecting rod into a rotational movement, and a bearing that rotatably supports the journal of the crankshaft. In the case of the crankshaft system, the vibrations transmitted from the connecting rod to the crankshaft and bearing are converted into noises.
Conventionally, the transmission of vibrations from vibration sources to various components was controlled by using vibration control material such as rubber and plastic in the path of vibration transmission from the vibrations sources in the valve actuating mechanism and crankshaft system and thereby attenuating the transmission of vibrations from the vibration sources to the various components. Such a prior attempt at reducing vibrations and noises in internal combustion engines is disclosed, for instance, in Japanese patent laid open publication 6-185522.
However, the vibration control material such as rubber and plastic has a poor resistance to deformation and prone to degradation as compared with metallic material such as an aluminum alloy which is typically used in various components of the engine.